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27-29 September 2023, University of Bayreuth (UBT)

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A large-volume air sampling system (LVASS) for isotope determination of atmospheric chloromethane

Rebekka Lauer1, Markus Greule1, Frank Keppler1
1 Institute of Earth Sciences, Heidelberg University, Im Neuenheimer Feld 234-236, 69120 Heidelberg, Germany

P 5.1 in Methods, Models, Standards

The global budget of chloromethane (CH3Cl) remains highly uncertain despite its significant role in catalyzing stratospheric ozone loss. Since conventional, concentration-based methods have limitations in improving current budget estimates including the balance of sources and sinks, stable isotopes offer an alternative approach to address these uncertainties. However, a crucial prerequisite for this concept is the determination of average tropospheric δ2HCH3Cl, δ13CCH3Cl and δ37ClCH3Cl values, which poses a considerable analytical challenge due to the low abundance of atmospheric CH3Cl of around ~550 parts per trillion by volume. Hence, we constructed a large-volume air sampling system (LVASS) capable of collecting sufficient amounts of CH3Cl from 50 to 500 L of air at three distinct locations in Germany: Heidelberg University (urban), Hohenpeißenberg (boundary layer) and Schneefernerhaus (free troposphere).

We present preliminary results of δ2HCH3Cl and δ13CCH3Cl values from the first sampling campaigns applying the LVASS at these three locations. The δ2HCH3Cl and δ13CCH3Cl values were measured by stable isotope ratio mass spectrometry. We will particularly focus on the reliability of the atmospheric measurements and the evaluation of potential fractionation processes associated with the LVASS, purification system and analysis by IRMS.

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